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From: Phil Hobbs on 6 Jan 2010 21:02
On 1/6/2010 5:47 PM, Jon Kirwan wrote:
> On Wed, 06 Jan 2010 16:33:13 -0500, Phil Hobbs
> <pcdhSpamMeSenseless(a)electrooptical.net> wrote:
>
>> On 1/6/2010 3:51 PM, Jon Kirwan wrote:
>>> On Wed, 06 Jan 2010 15:32:34 -0500, Phil Hobbs
>>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote:
>>>
>>>> <snip>
>>>> The easiest way to do that is to stick the current into the emitter of a
>>>> transistor, bias the base someplace reasonable--somewhere _very_ quiet,
>>>> 1 nV/sqrt(Hz) or less--and come out the collector...i.e. a common base
>>>> buffer.
>>>
>>>> <snip>
>>>
>>>>> So what does the common-base BJT do? Act as a
>>>>> current-follower, pinning the voltage of the emitters of the
>>>>> differential pair?
>>>>>
>>>> No, it's the other way up: low impedance in, high impedance out--the
>>>> collector goes to the diff pair emitters. You want the current not to
>>>> depend on the voltage at the emitters.
>>>
>>>> <snip>
>>>
>>> What about Early effect here? With the common base arranged
>>> collector feeding the diff pair emitters that are moving
>>> around? That would seem to move the collector and thus some
>>> difference due to the Early effect, to me. What am I
>>> missing?
>>>
>>> Jon
>>
>> The magnitude of the change. In general purpose NPNs, the Early voltage
>> is ~100V, whereas the emitters only move 50 mV at most, and the resistor
>> current sources won't be dropping anywhere near 100V. The OP can
>> probably use BF862 JFETs just as well, but I wouldn't lose too much
>> sleep over it, at least at the 60 dB cancellation level. If he needs
>> more than that, it'll start to matter, but so will a lot of other things.
>>
>> Cheers
>>
>> Phil Hobbs
>
> Got it. The diff pair bases can't be driven around that much
> so their shared emitters won't move much, etc.
>
> By the way, 100V seems to be kind of a "blind default" case
> (unmeasured, it seems to me, but estimated by some gross
> sweep of the hand) for modeling small signal NPNs like the
> 2N3904 or 2N2222. But I don't have much experience actually
> measuring the parameter -- something I should remedy. The
> specific NPNs the OP might use for the common base design
> might have significantly smaller magnitude Early voltage,
> though. Regardless, your point stands. I can't recall ever
> seeing a model (as I said, I've not measured so I'm lacking
> experience there) with an Early voltage less than 20V. And
> even there movement in the tens of millivolts shouldn't be a
> serious problem. (Maybe before I say that, I should see what
> 50mV movement does on a VAF=20V device against the 60dB
> level, though. At my level of understanding, this is kind of
> interesting to think about.)
>
> Thanks,
> Jon

I looked at the 2N3904, which has a typical Early voltage of 140V as
measured by Google. ;)

Quicker things like the HFA3046 are down in the 20V range, which can be
very inconvenient...I had a beautiful noise canceller front end blown
out of the water by that problem.

It used a non-monolithic diff pair, and the temperature tracking was
done by changing V_CE of one half of the diff pair so that the power
dissipation in the two sides was identical, regardless of the current
split ratio. It would have worked great, except that V_A was about
12V.... (John L. pointed that out to me, the rotter. ;) )

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: Tim Williams on 7 Jan 2010 00:03
"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message
news:654ak5d9slgfgo3ordpd98trrdriu5kf5e(a)4ax.com...
> By the way, 100V seems to be kind of a "blind default" case
> (unmeasured, it seems to me, but estimated by some gross
> sweep of the hand) for modeling small signal NPNs like the
> 2N3904 or 2N2222. But I don't have much experience actually
> measuring the parameter -- something I should remedy.

I just measured it the other day, as a matter of fact. (I'm finally to the
point in the curriculum where we're supposed to investigate and use
transistors... how droll!) I got 220V for this particular 2N3904 at 3.8mA
Ic. In contrast, the Multisim v10 Spice model has the unusually low value
of 74V.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


From: Jon Kirwan on 7 Jan 2010 00:28
On Wed, 06 Jan 2010 21:02:05 -0500, Phil Hobbs
<pcdhSpamMeSenseless(a)electrooptical.net> wrote:

>On 1/6/2010 5:47 PM, Jon Kirwan wrote:
>> On Wed, 06 Jan 2010 16:33:13 -0500, Phil Hobbs
>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote:
>>
>>> On 1/6/2010 3:51 PM, Jon Kirwan wrote:
>>>> On Wed, 06 Jan 2010 15:32:34 -0500, Phil Hobbs
>>>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote:
>>>>
>>>>> <snip>
>>>>> The easiest way to do that is to stick the current into the emitter of a
>>>>> transistor, bias the base someplace reasonable--somewhere _very_ quiet,
>>>>> 1 nV/sqrt(Hz) or less--and come out the collector...i.e. a common base
>>>>> buffer.
>>>>
>>>>> <snip>
>>>>
>>>>>> So what does the common-base BJT do? Act as a
>>>>>> current-follower, pinning the voltage of the emitters of the
>>>>>> differential pair?
>>>>>>
>>>>> No, it's the other way up: low impedance in, high impedance out--the
>>>>> collector goes to the diff pair emitters. You want the current not to
>>>>> depend on the voltage at the emitters.
>>>>
>>>>> <snip>
>>>>
>>>> What about Early effect here? With the common base arranged
>>>> collector feeding the diff pair emitters that are moving
>>>> around? That would seem to move the collector and thus some
>>>> difference due to the Early effect, to me. What am I
>>>> missing?
>>>>
>>>> Jon
>>>
>>> The magnitude of the change. In general purpose NPNs, the Early voltage
>>> is ~100V, whereas the emitters only move 50 mV at most, and the resistor
>>> current sources won't be dropping anywhere near 100V. The OP can
>>> probably use BF862 JFETs just as well, but I wouldn't lose too much
>>> sleep over it, at least at the 60 dB cancellation level. If he needs
>>> more than that, it'll start to matter, but so will a lot of other things.
>>>
>>> Cheers
>>>
>>> Phil Hobbs
>>
>> Got it. The diff pair bases can't be driven around that much
>> so their shared emitters won't move much, etc.
>>
>> By the way, 100V seems to be kind of a "blind default" case
>> (unmeasured, it seems to me, but estimated by some gross
>> sweep of the hand) for modeling small signal NPNs like the
>> 2N3904 or 2N2222. But I don't have much experience actually
>> measuring the parameter -- something I should remedy. The
>> specific NPNs the OP might use for the common base design
>> might have significantly smaller magnitude Early voltage,
>> though. Regardless, your point stands. I can't recall ever
>> seeing a model (as I said, I've not measured so I'm lacking
>> experience there) with an Early voltage less than 20V. And
>> even there movement in the tens of millivolts shouldn't be a
>> serious problem. (Maybe before I say that, I should see what
>> 50mV movement does on a VAF=20V device against the 60dB
>> level, though. At my level of understanding, this is kind of
>> interesting to think about.)
>>
>> Thanks,
>> Jon
>
>I looked at the 2N3904, which has a typical Early voltage of 140V as
>measured by Google. ;)

Well, the LTSpice 2n3904 (Philips, it says) shows VAF=100.
YMMV, of course.

>Quicker things like the HFA3046 are down in the 20V range, which can be
>very inconvenient...I had a beautiful noise canceller front end blown
>out of the water by that problem.

Okay. So it _could_ be a problem. Glad to see I'm not
totally confused.

>It used a non-monolithic diff pair, and the temperature tracking was
>done by changing V_CE of one half of the diff pair so that the power
>dissipation in the two sides was identical, regardless of the current
>split ratio. It would have worked great, except that V_A was about
>12V.... (John L. pointed that out to me, the rotter. ;) )
>
>Cheers
>
>Phil Hobbs

Egads. Maybe I should be glad I picked up some monolithic
pairs of both polarities. (I haven't used them for diff
pairs, as just current mirrors.)

Thanks,
Jon
From: Jon Kirwan on 7 Jan 2010 04:21
On Wed, 6 Jan 2010 23:03:18 -0600, "Tim Williams"
<tmoranwms(a)charter.net> wrote:

>"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message
>news:654ak5d9slgfgo3ordpd98trrdriu5kf5e(a)4ax.com...
>> By the way, 100V seems to be kind of a "blind default" case
>> (unmeasured, it seems to me, but estimated by some gross
>> sweep of the hand) for modeling small signal NPNs like the
>> 2N3904 or 2N2222. But I don't have much experience actually
>> measuring the parameter -- something I should remedy.
>
>I just measured it the other day, as a matter of fact. (I'm finally to the
>point in the curriculum where we're supposed to investigate and use
>transistors... how droll!) I got 220V for this particular 2N3904 at 3.8mA
>Ic. In contrast, the Multisim v10 Spice model has the unusually low value
>of 74V.

Can you describe more about how you did your measurement of
Va? I'm guessing you set Vbe to some voltage and Vce to
about the same and measured Ic at 3.8mA and then changed Vce
to some other higher voltage and measured Ic again, and then
computed the slope and divided that into 3.8mA to get your
figure. (Or multiplied, depending upon whether you computed
the slope as a resistance or conductance.) But I'm curious
if any of that is right. Could you describe the details?

In any case, it would seem also that calculations generating
large Va figures are often fraught with inaccuracies (not
that it matters for large Va values, since that means the
effect is nearing a negligible value, anyway.)

I'm curious if you tried this at a number of different Vbe's
to see if there was a basewidth modulation effect, too.

Thanks,
Jon
From: Tim Williams on 7 Jan 2010 09:37
"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message
news:iasak5pia917e27tnjh5bs601reh86qvpe(a)4ax.com...
> Can you describe more about how you did your measurement of
> Va?

Constant Ib. I can't imagine trying to take such a measurement at constant
Vbe, think of the runaway. However, I did not track change in Vbe vs. Vce,
so if it's defined in terms of constant Vbe, my data may be incorrect.

As for the slope, I measured Ic for Vce = 0...10V and got the
slope-intercept for points from 1-10V.

> In any case, it would seem also that calculations generating
> large Va figures are often fraught with inaccuracies (not
> that it matters for large Va values, since that means the
> effect is nearing a negligible value, anyway.)

That could very well be. Hey, I didn't get a negative number, so that's one
thing!

> I'm curious if you tried this at a number of different Vbe's
> to see if there was a basewidth modulation effect, too.

The Early effect is explained in terms of base width modulation from the B-C
depletion region, and has nothing to do with the B-E junction itself. I
don't know why Vbe might change (except to secondary effects like Ie and
improved beta).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


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